Equine Narcolepsy

As Jennifer Johnson stood in the lineup awaiting the judge's decision with her recently acquired Quarter Horse mare Dobie Breeze, the last thing she expected was that her mount suddenly would collapse in a heap beneath her. But that's just what happened. Moments later, the mare scrambled back onto her feet and calmly resumed her normal stance, while her startled and perplexed owner looked on. Fortunately for Jennifer, the mystery of the mare's strange behavior was solved practically on the spot when a spectator informed her that her horse appeared to be narcoleptic. Research confirmed the diagnosis in the following days.

Although the episodes continued, Jennifer showed Dobie successfully for many years after that initial incident (the mare earned a place on the AQHA Register of Merit). During events that required a lot of standing still, such as showmanship and halter classes, she learned to spot the signs--drooping eyelids, a slight lowering of the head--in time to give the mare a pinch or rattle the reins and snap her back to full consciousness. When picking out her tail, Jennifer would remain watchful and ready to jump out of the way in case Dobie started to fall. Crashing sounds during the night often summoned Jennifer to the stable. There, she would find Dobie standing calmly, often with a few new scrapes and scuffs acquired from a dive into the manger minutes earlier.

Dobie Breeze, now retired from showing, was fortunate to fall into the hands of a kind, patient owner who was willing to work with this otherwise healthy and athletic horse throughout a lifetime that would be oddly punctuated with attacks of equine narcolepsy.

One must remember that equine narcolepsy is an extremely dangerous disease, and only professional horse people should work with these horses. The consequences of an attack can be devastating and life threatening to the person around a narcoleptic horse.

Narcolepsy, The Disease

Narcolepsy is a rare and severe sleep disorder that has been reported in many mammals. In humans, it affects approximately one to two in 2,000; in horses the incidence probably is even lower. Although narcolepsy was identified in humans almost 100 years ago, like many sleep disorders research was not initiated until the early 1970s. All that we know of sleep disorders in animals stems from studies on the condition in humans, and even today relatively little time and money is available for research on sleep disorders. Ironically, it is estimated that the consequences of sleep disorders have a larger impact on the North American economy than any other physical affliction, mainly due to people missing work.

The causes of narcolepsy remain largely a mystery, although scientists have come to understand something of the processes involved. Sleep was once thought to be nothing more than a simple slowing down of the brain and body. We now know that sleep and its various phases are a complex orchestration of neurochemicals, hormones, biological clocks, and various body chemicals and systems. We also know that certain neurotransmitters--such as adrenaline, serotonin, and dopamine--play a large role in gov-erning the sleep process and its patterns.

A neurotransmitter is a chemical manufactured by the nervous system and stored in the body's nerve cells. When a nerve cell releases a neurotransmitter (fires), an electrochemical reaction takes place. In narcolepsy, the neurotransmitters related to sleep fire improperly or off-cue. Some theories about the cause of narcolepsy blame depleted levels of neurotransmitters, or the body's inability to produce sufficient amounts of them.

Studies of narcolepsy in humans have revealed that there are four components of the condition. The first component is excessive daytime drowsiness, during which periods of rapid eye movement (REM) sleep might invade waking consciousness. REM sleep is the phase of sleep during which we dream. It is so named because the eyes move rapidly back and forth during this period. The second component is cataplexy, which is a sudden and temporary loss of muscle tone while conscious.

Cataplexy is one of the most baffling aspects. In humans, an attack might be triggered by sudden surges of emotion, especially laughter, anger, or surprise, causing victims to buckle at the knees and collapse. After a few moments, they can stand again.

Sleep paralysis is the third component, and perhaps the most frightening for victims. During sleep paralysis, a narcoleptic might awaken during the night and find that he or she is totally paralyzed. The paralysis usually lasts no longer than a minute but might seem to last for hours to the victim. Daytime hallucinations and/or extremely terrifying dreams are the fourth component.

Narcolepsy In Horses

The first two components of human narcolepsy (the sudden onset of REM sleep and cataplexy) have been witnessed in horses. As horses are unable to tell us whether they dream, hallucinate, or experience sleep paralysis, we will never know if the other two components play a part or not.

In narcoleptic horses, the sudden onset of REM sleep usually occurs during periods of inactivity--while the horse is in his stall, at pasture, or standing relaxed in a run-in shed. Typically, the horse will drop his head quite low and spread his front legs for balance while the hindquarters sag. The horse's eyes might be closed or half-open, and eye movement (REM) is visible. Most horses remain standing during attacks, but some, particularly ponies, sit down on their haunches and might roll onto their sides. Attacks can last from a few seconds to 10 minutes. Some horses can be aroused easily, others not at all. In all cases, horses awake calmly and resume their normal behavior immediately as if nothing happened.

Cataplexy is a more problematic and potentially dangerous aspect of narcolepsy in horses, simply because of the size of the animal. While emotions have proven to be attack-triggers in humans, simple actions such as stroking the head and neck, leading a horse out of his stall, hosing or bathing after exercise, or the intake of the first mouthful of water or grain at feeding time, can cause a narcoleptic horse to buckle suddenly at the knees, stumble, and possibly collapse. As in humans, the cardiac and respiratory muscles continue to work normally during the episode, while all other muscle groups lose function to varying degrees. Forcing a horse to try to walk his way out of a cataplectic episode can cause him to become ataxic (lose control of all muscle functions). The best approach is simply to stand back and let the episode run its course. Within minutes, or even seconds, the horse will regain control and return to normal.

Most literature on equine narcolepsy agrees that episodes, particularly cataplectic episodes, do not occur during exercise; however, reports from owners differ. Jennifer Johnson's mare Dobie had two foals. One of them, now an eight-year-old gelding, is a narcoleptic who regularly collapsed under saddle at slower gaits. However, the faster he went, the less likely he was to suffer an attack. His owners decided to take a pragmatic approach and turn him into a barrel racer, and have had few problems since.

Accounts such as this one suggest that narcolepsy is a genetic disorder and a heritable one. This might explain why certain breeds, such as Thoroughbreds, Quarter Horses, Standardbreds, Morgans, Appa-loosas, and Welsh ponies, appear to be more susceptible to narcolepsy than other breeds. Narcolepsy also has been known to run in certain families of Suffolks and Shetlands in the United Kingdom, where the condition is sometimes referred to as "fainting disease."

In horses, as in humans, narcolepsy generally declares its presence early in life. The first episodes often occur in foals around six months of age, although adult-onset cases have been reported. Sometimes foals can outgrow it, but often it is with them for life. A predictable pattern to the attacks, including their frequency and duration, usually is established within two weeks of the initial episode.

Diagnosis And Treatment

One of the most troubling aspects of narcolepsy is that it can be mistaken for other conditions that are much more serious and require immediate intervention--such as shock, poisoning, snake bite, botulism (especially in foals), heat stroke, seizures, or exhaustion. Narcolepsy can strongly resemble hyperkalemic periodic paralysis (HYPP--a genetic muscle stimulation disorder that causes varying levels of intermittent paralysis) in Quarter Horses. That certainly would elicit panic in a horse owner. Narcolepsy is rare enough that most vets have not had any experience with the condition, and since attacks usually occur during rest and are short-lived, it is rare that veterinarians get to witness the episodes. However, knowledgeable, well-read veterinarians will be able to make a diagnosis based on a detailed account of the horse's actions and physical attitude.

If there is serious doubt, a test involving the administration of physostigmine salicylate can be done by a veterinarian. In narcoleptic horses, physostigmine will cause a cataplectic attack within three to 10 minutes of administration. The test also can induce a colic attack, so it only should be used when other life-threatening conditions cannot be ruled out by any other means.

There is no cure for narcolepsy in humans or horses, and treatments for the disease can cause a multitude of other problems. In human patients, the disease can be curtailed with lifestyle adjustments to reduce stress, combined with the well-timed use of stimulants and antidepressants. Knowledge of how the disease works and learning to identify possible triggers can help patients dramatically in coping with and avoiding attacks. Obviously, such approaches would not work with equine patients.

On the drug therapy side, imipramine, a tricyclic antidepressant, has been used to treat narcoleptic horses with inconsistent results, while atropine sulfate has been shown to relieve the condition in horses for up to 30 hours. Both drugs can cause severe colic attacks, so the risks far outweigh the benefits.

There is some good news to report in drug therapy options. Recently, Sharon Doolittle, DVM, whose practice is based in Johnston, R.I., has had success treating narcoleptic horses with the antidepressant amitriptylene. Doolittle treated four horses with the drug with excellent results in three of them (the fourth horse was sold before she could make an assessment). In each of the cases, the owners reported that their horses would become drowsy, suddenly fall asleep, then crash to the ground while standing in cross-ties, being groomed, waiting between classes at horse shows, or standing in their paddocks. The owners were concerned that their horses might injure themselves, perhaps severely, by collapsing so suddenly. Doolittle did not witness any of the incidents, and all of the owners declined the physostigmine test, fearing side effects.

After doing blood work (a complete blood cell count and chemical profile) and considering the owners' descriptions, she made presumptive diagnoses of narcolepsy and began administering amitriptylene, a drug that had been recommended to her by researchers at Tufts University. One horse responded with a complete cessation of symptoms. A second horse responded with 80-90% f ewer epi-sodes, while a third horse had to be given the maximum dosage to achieve a 50-60% reduction in the fre-quency of the attacks. While amitriptylene can cause colic, none of the horses treated by Doolittle experienced negative side effects.

An owner's decision to use drug therapy should be made only after thorough consultation with a veterinarian. The frequency and severity of the attacks, the potential for injury, and the risks of drug treatment must be weighed before deciding on the best course of action. In some cases, the wisest approach might be to follow Jennifer Johnson's lead and simply learn to live with it.

With or without drug therapy, narcoleptic horses will require a little more monitoring and attention to their environment than normal horses. If you suspect you've got a "sleeper" on your hands, consider these management tips:

Invest in break-away snaps for your lead shanks and cross-ties.

Install rubber padding on the walls and floor of the horse's stall.

Remove any objects from the paddock, stable, and stall that could injure the horse if he were to collapse on or near them.

If you don't already know how to tie a quick release knot, learn now.

A team of researchers at Stanford University has identified the gene that causes narcolepsy in dogs (Lin, L.; Faraco, J.; Li, R.; Kadotani, H.; Rogers, W.; Lin, X.; Qiu, X.; de Jong, P.J.; Nishino, S.; Mignot, E. The Sleep Disorder Canine Narcolepsy Is Caused by a Mutation in the Hypocretin (Orexin) Receptor 2 Gene. Cell, 98 (3); 365-376). The result of a 10-year study, this breakthrough has enormous implications and might prove to be a crucial step in the development of new treatments and cures for various sleep disorders in humans and animals.

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